Is yellow bowel movement a possible side effect of taking elavil (amitriptyline)?

Not likely. Yellow stools are generally a sign of fat that hasn't been absorbed but passed through. If anything, Elavil (amitriptyline) will cause constipation, slowed intestinal passage and increased fat absorption, so darker stools. If this condition is truly noticeable, different and persistent, you should speak to a GI expert--and of course let your prescriber know!